Formation of the placenta
Differentiation of the trophoblast

Trophoblast cells coming into contact with maternal tissue during adplantation (S4) and implantation (S5) lose their cell boundaries and fuse together to form a syncytium (syncytiotrophoblast). In the internal layer, individual cells remain separated (cytotrophoblast). During further penetration into the uterine mucosa, the syncytiotrophoblast increases in mass (S5a, solid trophoblast or prelacunar stage). It proliferates mainly at the embryonic pole. However, the remaining regions of the blastocyst are also surrounded by the syncytium once implantation is complete.